1. Field of the Invention
The invention relates to a fuel cell, especially a methanol fuel cell, and to a method of operating this fuel cell.
2. Description of the Related Art
A fuel cell has a cathode, an electrolyte and an anode. The cathode is supplied with an oxidizing agent, for example, air or oxygen, and the anode is supplied with a fuel, for example, hydrogen or methanol.
Various fuel cell types are known, for example, the SOFC fuel cell (SOFC=solid oxide fuel cell) from the publication DE 44 30 958 C1 and the PEM fuel cell (PEM=proton exchange membrane) from the publication DE 195 31 852 C1.
The operating temperature of a PEM fuel cell is about 80xc2x0 C. A PEM fuel cell can in principle be either an acid or alkaline fuel cell, depending upon the type of membrane or the working medium. Usually protons are formed at the anode of a PEM fuel cell having a proton conductor in the presence of the fuel by means of a catalyst. The protons pass through the electrolyte and combine at the cathode side with oxygen arising from the oxidation medium to water. Electrons are thereby liberated and electrical energy is generated. The drawback of a methanol fuel cell with a proton conductor is that the protons, under the influence of the electric field, in their solvate shells entrain water molecules along with them. This electrophoresis effect is associated with a very high drag factor (number of entrained water molecules per proton). This means on the one hand that too much water is transported from the anode to the cathode which has a disadvantageous effect on the thermal balance; on the other hand, methanol is entrained which in general can form a mixed potential at the cathode and result in a significant reduction in power.
Multiple fuel cells are as a rule connected together electrically and mechanically to produce large electric power utilizing connecting elements. These arrangements are called fuel cell stacks. For the fuel, methane or methanol, among others, can be used. The mentioned fuels are converted by reformation or oxidation to, among other things, hydrogen or hydrogen-rich gas.
There are two types of methanol fuel cells. The so-called indirect methanol fuel cell in which initially in a preceding process step a hydrogen-rich gas mixture is produced and which is then led into a polymer electrolyte fuel cell of the usual hydrogen type with anodic platinum ruthenium catalysts. This process variant is then comprised of two stages: gas production and the usual fuel cell. A further significantly simpler variant from the point of view of process technology, is the so-called direct methanol fuel cell (DMFC) in which the methanol, without intervening stages from the process technology point of view, is directly fed to the fuel cell. This cell has in comparison to the first, however, the disadvantage that with a proton conductor as an acidic medium, the direct electrochemical oxidation of methanol is a kinetically strongly limited process which, with reference to a fuel cell, gives rise to considerable loss of cell voltage. Even with the best results with the DMFC cells to date these cells hardly can be expected to compete in classical configurations with the indirect methanol fuel cell.
This can as a first instance be due to the fact that both the methanol permeation rate and the water vapor enthalpy in the cathode compartment are too high in the case of the present day cells. Furthermore, because of the unsatisfactory methanol oxidation rate, it is necessary for the operating temperature of the cell to be significantly above 100xc2x0 C. There is however no appropriate electrolyte which can remain functional above 120xc2x0 C.
To be economical relative to indirect methanol cells, the DMFC must have voltages smaller by only 100 mV at the same current densities by comparison to the indirect cells (with MeOH-permeation) or around 150 mV smaller without permeation. As simulation results show, the greatest loss originates in anodic overvoltage which derives from the highly irreversible electrode kinetics. For that reason, the catalyst coating must also be uneconomically high; because of the methanol permeation the cathodic catalyst coating must be 10 times higher than is the case with hydrogen cells.
From W. Vielstich: Brennstoffelemente (Fuel elements), Verlag Chemie, 1965, P. 73-91 it is known as state of the art to provide an alkali methanol oxidation in a fuel cell. This method has the advantage, by comparison to the known acid variant, that the electric chemical reactions run far more quickly and thus the power of the fuel cell is significantly higher. In practical applications, KOH is used which is immobilized by a diaphragm in the fuel cell. It has, however, been known for a long time from W. Justi, A. Winsel; Kalte Verbrennung (Cold Combustion), Fuel Cells, Franz Steiner Verlag, Wiesbaden, 1962 that in this process, carbonate is formed as a drawback and can give rise as a rule to plugging up of the diaphragm and to a significant reduction in the conductivity among other things of the carbonate electrolyte in the diaphragm. Furthermore, problems cannot be excluded in the three-phase zone of the catalytic layer of the fuel cell electrode because of carbonate formation.
It is the object of the invention to provide a fuel cell for the conversion of methanol which is effective and can avoid the aforedescribed drawbacks. It is also an object of the invention to provide a method of operating such a fuel cell.